By letting police and prosecutors decide when to enforce the law and when not to, you make them defacto judges. When their decisions carry penalties, they become defacto "executioners" (Yes, I put the word executioners in quotes because they do not actually kill you, but they do enact punishments including jail time and financial costs).
When you have laws the get selectively enforced, innocent people are already getting punished. Worse yet, you are giving a criminal the tool to use that punishment for his own agenda. Selective enforcement means that the law isn't really a law, it is just an excuse to use the color of law to abuse citizens, and lives are already being ruined by stupid laws combined with selective enforcement.
A big reason that we have so many stupid laws is due to selective enforcement.